In projectors which use phosphors for light generation, it is conventional to use a laser and a rotating phosphor wheel arranged downstream of the laser (LARP: Laser Activated Remote Phosphor). The phosphor wheel typically includes a central driveshaft, which is driven by a motor.
The laser generates primary light, which is at least partially converted by phosphors located on the phosphor wheel into secondary light of longer wavelength (down-conversion). The phosphor wheel typically rotates with a constant angular velocity under a spatially fixed light spot of the primary light beam. By sufficiently rapid sequencing of the light components, the human eye can no longer resolve the individual color components and combination light is essentially generated, the sum color locus of which is given by a superposition of color loci of the individual color components. A component of the individual color components of primary light and secondary light is dictated primarily by an angular sector length of the phosphors on the phosphor wheel. Such projectors are known in principle and need not be described further here.
When the phosphor wheel is exposed, a certain power loss is generated in the phosphor, which is based for the most part on the so-called Stokes shift (which represents an energy difference of the photos between the absorbed and emitted radiation). This power loss leads to heating of the phosphor. The heat loss is dissipated by convection, radiant cooling and by thermal conduction of the carrier material of the phosphor wheel to the motor axle of the phosphor wheel, so that a stable operating temperature is established. Many phosphors (for example nitridic phosphors for the red spectral range) exhibit a conversion efficiency which is strongly dependent on an operating temperature. In order to increase and/or scale the output power of such projectors, a primary light power may be increased, although this also increases the heat loss. The operating temperature of the phosphor and of the phosphor wheel is thereby in turn increased. Owing to the temperature dependency of a conversion efficiency of phosphors, the overall efficiency of the light generation decreases with an increasing power.
This has previously been compensated for by increasing a wheel diameter of the phosphor wheel, since a larger surface area is then available for the thermal dissipation, and a reduction of the average power density is made possible. For instance, the wheel diameter is often increased from 33 mm to 40 mm, and now 55 mm or even 70 mm. Owing to the requirements in terms of the installation space and also in terms of the power of a motor of the phosphor wheel, however, there are limits to this concept.
As an alternative, a reduction of the average power loss can be achieved by generating a red light component not through wavelength conversion by means of a phosphor, but by means of a red light-emitting diode (LED). However, because of the luminous density of LEDs, which is limited in this case, a light flux increase to several thousand lumens is not possible, or is possible only with significant enlargement of an imaging unit, which is in turn associated with a superproportionally large cost increase.